The Gobi Desert is characterized by flat plains. Great areas areunderlain by glacial dust (loess) blown in from glaciated partsof Asia in the recent geological past (the last 1-2 millionyears). Smaller areas are covered by coarser material (sand),and although sand is moved shorter distances, it is neverthelessheaped up by sand-moving winds into organized lines of dunes, asseen in this near-vertical view. Dune fields such as this usual-ly occupy low topographic points into which rivers have drainedfor millions of years. In such basins, water is sometimes nearthe surface. In this photograph, brackish pools occupy hollowsbetween the lowest-lying dunes. Brackish water and an almostcomplete lack of rainfall prevent the growth of vegetation.(S27-39-44)